The present invention relates to watthour meters and, more particularly, to current stators for watthour meters and their interface with the remainder of the meter.
Conventional electro-mechanical watthour meters employ a conductive metal disk rotated as the rotor of a small induction motor by interaction with fluxes generated by opposed voltage and current stators. When the fluxes produced by the voltage and current stators are in quadrature, the rotational torque experienced by the disk is proportional to the voltage applied to the load multiplied by the current consumed by the load; that is, the power consumed by the load. Disk rotation is magnetically resisted in proportion to its rotational speed. Thus, the disk speed is proportional to the power consumed by the load. Each rotation of the disk represents a predetermined increment of energy consumed. The rotations of the disk are accumulated over time in a mechanical or electronic accumulator, or register, for billing purposes by the utility supplying the power.
Although watthour meters are precision measuring instruments, they are manufactured in a volume measured in the millions per year. Several manufacturers are engaged in the business of manufacturing watthour meters and such manufacturers keenly compete for the business. Therefore, for a manufacturer to maintain its position in the industry, it is imperative that it find more efficient ways to produce such watthour meters.
Manufacturing efficiency is improved by reducing the number of different types of parts which must be manufactured. For example, two different parts may be replaced by two identical parts, each capable of performing the function of the part it replaces. This assists in several ways. First, only a single part must be designed. Second, manufacturing runs of twice the length are possible to produce the desired amount of identical parts rather than two shorter runs to produce the two different parts. Third, the machine setup time between runs to produce two different parts, during which the manufacturing asset is idle, is eliminated. Finally, two separately catalogued and stocked parts are replaced with a single catalog and stock item.
It is conventional in polyphase watthour meters to provide two or three sets of opposed current and voltage stators spaced about the circumference of the meter disk to drive the meter at a speed proportional to the total power delivered to the load through all three phases of a three-phase line. At a minimum, a first set of voltage and current stators is disposed at the right of the meter, as viewed from the front, and a second set of voltage and current stators is disposed at the left of the meter. When a third set of voltage and current stators is used, they are disposed at the rear of the meter, angularly spaced midway between the left and right sets of voltage and current stators. For convenience in the following description, these sets of voltage and current stators are referred to as the right, left and rear sets.
The style of watthour meter with which the present invention is principally concerned is one which plugs directly into a socket behind it. This is effected by connector blades extending rearward from the meter which are inserted into contacts in the socket. In order to permit interchangeability between meters of different manufacturers, the external interface between all socket-style watthour meters is rigidly specified by the American National Standard Institute, hereafter ANSI. That is, the dimensions and spacing of the connector blades are beyond the control of the meter manufacturer. The internal arrangement of a watthour meter, in contrast, is subject to performance specifications but the physical arrangement is generally under the control of the manufacturer.
The left and right current stators in a two-stator meter are generally mirror images of each other. They are positioned about 180 degrees apart around the circumference of the disk. Since they are mirror images, the left and right current stators are different parts requiring separate design, manufacture, catalog and stock. Substantial economies would result from replacing two different current stators with two identical current stators while maintaining rigid adherence to the ANSI interface standard.